1. Field of the Invention
The present invention relates to an apparatus and method for recognizing position information of a mobile station (MS) (i.e., a mobile terminal) in a mobile communication system, and more particularly to an apparatus and method for calculating satellite acquisition information to detect position information of the MS in a Network Assisted GPS (Global Positioning System).
2. Description of the Related Art
Generally, technologies for determining a position of a target object (e.g., an MS) using a GPS satellite have been widely used in a variety of applications, for example, navigation systems for vehicles or ships. A GPS receiver for use in the above technologies receives a plurality of satellite signals containing satellite position coordinate information from a satellite, and detects a pseudo range between the satellite and the GPS receiver, such that it can calculate its own current position using the pseudo range and the satellite position coordinate. A conventional GPS receiver independently calculates position information of a target object without communicating with an external device. These conventional GPS receivers, therefore operate in what has generally been called a standalone GPS scheme. Most GPS receivers have generally used the stand-alone GPS scheme.
There has recently been proposed a method for embedding the GPS receiver in the MS. However, a battery built in the MS has a limited amount of available electric energy, such that the GPS receiver built in the MS is designed to be operated only in the specific case where a position determination operation is requested. It takes a long period of time for the built-in GPS receiver to make a position determination once an MS's position measurement request has been made. As a result, the GPS receiver built into the MS it cannot immediately provide a user with current position information upon receiving the position measurement request from the user.
In order to solve the aforementioned problems, there has recently developed a network assisted GPS system. This network assisted GPS system has a GPS receiver installed (hereinafter referred to as a reference station GPS receiver) at a fixed position, such that it can always receive a satellite signal containing specific information of a satellite via the reference station GPS receiver. Upon receiving a position measurement request signal from the MS, the network assisted GPS system transmits specific information collected by the reference station GPS receiver to the MS, such that the MS can calculate its own position using the received information within a short period of time (e.g., 10 seconds).
There are three components of the information transferred from the network assisted GPS system to the MS. These include an observable GPS satellite number (e.g., a GPS satellite's pseudo random number (PRN), code phase and pseudo range search window (PRSW) information. The PRSW information corresponds to pseudo range information of individual GPS satellites, Doppler frequency and frequency search window (FSW) information corresponding to velocity information of individual GPS satellites.
Information applied to the MS is called satellite acquisition information. The satellite acquisition information can be acquired by processing an output signal of the reference station GPS receiver. A device for acquiring the satellite acquisition information is called a position determination entity (PDE).
The Network Assisted GPS system is shown in FIG. 1.
Referring to FIG. 1, an MS (Mobile Station) 100 can wirelessly communicate with a mobile communication base station (hereinafter referred to as an BS) 110, and can calculate its own position because it contains a GPS receiver. The PDE 120 can communicate with the BS 110, and includes a reference station GPS receiver 130. Although the PDE 120 is connected to the BS 110 in FIG. 1, it may also be connected to an mobile switching center (MSC). The BS 110 is composed of a base station transceiver subsystem (BTS) and a base station controller (BSC). It is assumed that the PDE 120 is connected to the BSC contained in the BS 110 in FIG. 1.
FIG. 2 is a conceptual diagram illustrating operations of the network assisted GPS system shown in FIG. 1.
Referring to FIGS. 1 and 2, if a user selects a position measurement command by pressing a predetermined button of the MS to recognize the user's position information, the MS 100 recognizes the user's position measurement command at step 200, and requests satellite acquisition information from a corresponding BS 110 at step 210. The BS 110 informs the PDE 120 of the fact that the MS 100 has requested satellite acquisition information at step 220.
Upon receiving the satellite acquisition information request signal (step 220), the PDE receives GPS satellite orbital information from the reference station GPS receiver 130 at step 230. The PDE 120 calculates satellite acquisition information to be applied to the MS 100 upon receiving the GPS satellite orbital information from the reference station GPS receiver at step 240. The PDE 120 transmits the calculated satellite acquisition information to the BS 110 at step 250. The BS 110 transmits the received satellite acquisition information to the MS 100 at step 260.
The network assisted GPS system has been estimated to be the most effective system capable of providing position information of the MS, and has already been standardized. A representative example of the network assisted GPS system has been disclosed in a Telecommunications Industry Association paper, entitled “Position Determination Service Standard for Dual-Mode Spread Spectrum Systems”, TIAIELA/Interim Standard 801(IS-801), the entire contents of which are herein incorporated by reference, by inventor Lake Louise in the province of Alberta, Canada on October 1999. Another example thereof has also been disclosed in a Telecommunications Industry Association paper, entitled “Enhanced Wireless PN-3890 911 Phase 2”, the entire contents of which are herein incorporated by reference, which has been published as J-STD-xxx in February, 2000. Most mobile communication service providers currently attempt to adapt the network assisted GPS system as an MS position determination system.
In the above-described network assisted GPS system, the PDE 120 always receives satellite orbital information from the reference station GPS receiver 130, and transmits satellite acquisition information calculated by the received satellite orbital information to the MS 100 upon receiving a request from the MS 100.
The satellite acquisition information transferred to the MS 100 is calculated based on the MS 100's estimated GPS signal search time (Ta) for determining a position of the MS 100, instead of using satellite orbital information transferred from the reference station GPS receiver 130 to the PDE 120 at a current time (Tc). Particularly, there is variation in code phase and Doppler shift information contained in the satellite acquisition information due to the movement of satellites with the lapse of time. Therefore, the code phase and Doppler phase of individual satellites must be calculated considering the time Ta at which the MS 100 searches for the GPS signal to recognize its position. In this case, the code phase is equal to a phase difference between the satellite and the MS 100 at the time Ta, and is calculated by a pseudo range between the MS 100 and the GPS satellite. The Doppler shift is equal to a frequency variation of the moving satellite.
The PDE 120 calculates position and velocity information of the satellite using satellite orbital information received from the reference station GPS 130 in such a way that it can calculate the code phase and the Doppler shift. In this case, the code phase is calculated using the satellite's position information, and the Doppler shift information is calculated using the satellite's velocity information. A representative example for controlling the PDE 120 to calculate the position and velocity information of the satellite has been disclosed in GPS standard positioning system (SPS) Signal Specification (2nd Edition, 2 Jun. 1995), the entire contents of which are hereby incorporated by reference.
The satellite position information can be calculated using the following Equation 1:xk=xp cos Ωk−yp cos ik sin Ωk yk=xp sin Ωk+yp cos ik cos Ωk zk=yp sin ik  [Equation 1]
where (xk, yk, zk) is position coordinate information of a k-th satellite, xk is an Earth-Centered, Earth-Fixed (ECEF) X-axis coordinate after the lapse of a predetermined time tk indicative of time variance, yk is a ECEF Y-axis coordinate after the lapse of tk, zk is a ECEF Z-axis coordinate after the lapse of tk, xp is a satellite position in an orbital plane, yp is a satellite position in an orbital plane, Ω is an argument of perigee, and ik is a variation in inclination angle of a satellite orbit after the lapse of tk.
The satellite time can be corrected using the following Equation 2:Δtsv=af0+af1tc+af2tc2+Δtr−TGD  [Equation 2]
where Δtsv is a time difference between the satellite and the MS, TGD is an estimated group delay difference, Δtr is a relative correction time, and af0˜af2 are identifiable satellite clock correlation values.
A satellite's velocity can be calculated using the following Equation 3:vx=−Ωk′yk+sin Ωk(zkik′−yp′ cos ik)+xp′ cos Ωk vy=Ωk′xk+cos Ωk(−zkik′+yp′ cos ik)+xp′ sin Ωk vz=ypik′ cos ik+yp′ sin ik  [Equation 3]
where vx is a velocity component in the X-axis direction, vy is a velocity component in the Y-axis direction, and vz is a velocity component in the Z-axis direction.
A satellite's acceleration can be calculated using the following Equation 4:ax=−Ωk′yk′+sin Ωk(zk′ik′−Ωk′xp′+ypik″ sin ik−yp″ cos ik+ik′yp′ sin ik)+cos Ωk(xp″+ypΩk′ik′ sin ik−Ωk′yp′ cos ik)ay=Ωk′xk′+cos Ωk(−zk′ik′+Ωk′xp′−ypik″ sin ik+yp″ cos ik−ik′yp′ sin ik)+sin Ωk(xp″+ypΩk′ik′ sin ik−Ωk′yp′ cos ik)az=sin ik(−ypik′2+yp″)=cos ik(ypik″+2ik′yp′)  [Equation 4]
where ax is an X-axis acceleration component of the satellite, ay is a Y-axis acceleration component of the satellite, and az is a Z-axis acceleration component of the satellite.
FIG. 3 depicts general satellite orbital information items to be managed by the PDE to calculate position and velocity information of the satellite. In order to Calculate the position and velocity information of the satellite, the PDE 120 must unavoidably manage a large amount of satellite orbital information shown in FIG. 3 in association with individual observable satellites, resulting in an ineffective system.
A method for calculating the Doppler shift and the code phase will hereinafter be described with reference to FIGS. 1 to 3. A pseudo range ρ between the satellite and the MS is calculated to calculate the code phase. The pseudo range ρ between the satellite and the MS can be calculated by substituting a position coordinate of the satellite and a coordinate of the MS into the following Equation 5:ρ=√{square root over ((xk−x)2+(yk−y)2+(zk−z)2)}{square root over ((xk−x)2+(yk−y)2+(zk−z)2)}{square root over ((xk−x)2+(yk−y)2+(zk−z)2)}−cΔtsv  [Equation 5]
where (x, y, z) is coordinate information of a BS communicating with the MS, and (xk, yk, zk) is satellite coordinate information calculated by Equation 1. In greater detail, it may be considered that the position of the MS is almost equal to that of the BS 110 communicating with the MS from the viewpoint of the satellite. Therefore, coordinate information of the MS is replaced with the other coordinate information (x, y, z) of a corresponding BS 110 so that a pseudo range can be calculated. The second term of the Equation 5 is adapted to correct the time difference Δtsv between the satellite and the MS, and the reference character “c” of the Equation 5 is the velocity of light.
The code phase to be calculated using satellite acquisition information is a parameter associated with time. The calculated pseudo range relates to a range (i.e., a distance), such that the code phase can be calculated on the condition that the calculated pseudo range is converted into time-dependent information (i.e., time-unit information). Therefore, the code phase can be represented by the following Equation 6:SV_CODE—PH=floor((ρ/c)*1000−t)*1023);t=floor((ρ/C)*1000)  [Equation 6]
where floor(x) is a function indicative of the highest integer of less than a real number x, SV_CODE_PH is a code phase between the satellite and the MS, and “t” is a specific value acquired by converting a pseudo range into a time-unit value.
The Doppler shift information is calculated using the velocity of the satellite. Doppler effect phenomenon is a specific state during which a frequency of a radio signal transmitted from a transmitter is different from that of a radio signal received in a receiver due to the movement of the transmitter and the receiver. A Doppler value transferred from the PDE to the MS as satellite acquisition information is called a pseudo Doppler value {tilde over (d)}. The pseudo Doppler value can be calculated using the following Equation 7:
                                                                        d                ∼                            =                                                d                  -                                      Δ                    ⁢                                                                                  ⁢                                          f                      T                                                        +                                      Δ                    ⁢                                                                                  ⁢                                          f                      R                                                                      =                                                                            -                                              f                        T                                                              ⁢                                          1                      c                                        ⁢                                                                                          r                                                                    ′                                                        -                                      Δ                    ⁢                                                                                  ⁢                                          f                      T                                                        +                                      Δ                    ⁢                                                                                  ⁢                                          f                      R                                                                                                                                              =                                                                    -                                          f                      T                                                        ⁢                                      1                    c                                    ⁢                                                            (                                              v                        -                                                  u                          ′                                                                    )                                        ·                                          r                                                                      r                                                                                                                    -                                  Δ                  ⁢                                                                          ⁢                                      f                    T                                                  +                                  Δ                  ⁢                                                                          ⁢                                      f                    R                                                                                                          [                  Equation          ⁢                                          ⁢          7                ]            
where d is a Doppler value, {tilde over (d)} is acquired by correcting the Doppler value d using an error value ΔfT−ΔfR, ΔfT is a variation in frequency of a satellite signal transferred from the satellite, ΔfR is a variation in frequency of a satellite signal received in the MS, v is a vector of a satellite velocity (vx, vy, vz) a is a vector of a satellite acceleration (ax, ay, az), u is a vector of an MS position (x, y, z), u′ is a velocity vector of the MS, and r is a difference between the satellite position vector (xk, yk, zk) and the MS position vector (x, y, z). Provided that the MS enters a halt state and is minimally affected by ΔfT and ΔfR, the above Equation 7 can be represented by the following Equation 8:
                                                                        d                ∼                            ≈                            ⁢                                                -                                      f                    T0                                                  ⁢                                  1                  c                                ⁢                                  v                  ·                                      r                                                                r                                                                                                                                                                    =                            ⁢                                                -                                      1                    λ                                                  ⁢                                                                                                    v                        x                                            ⁢                                              (                                                                              x                            k                                                    -                          x                                                )                                                              +                                                                  v                        y                                            ⁡                                              (                                                                              y                            k                                                    -                          y                                                )                                                              +                                                                  v                        z                                            ⁡                                              (                                                                              z                            k                                                    -                          z                                                )                                                                                                                                                                          (                                                                                    x                              k                                                        -                            x                                                    )                                                2                                            +                                                                        (                                                                                    y                              k                                                        -                            y                                                    )                                                2                                            +                                                                        (                                                                                    z                              k                                                        -                            z                                                    )                                                2                                                                                                                                                    [                  Equation          ⁢                                          ⁢          8                ]            
The Doppler shift indicative of a variation in wavelength (i.e., frequency) created by the satellite's movement while orbiting the earth can be calculated using the following Equation 9:
                                                                                          d                  ′                                ∼                            ≈                            ⁢                                                -                                      f                    T0                                                  ⁢                                  1                  c                                ⁢                                                                                                    (                                                                              a                            ·                            r                                                    +                                                                                                                  v                                                                                      2                                                                          )                                            ⁢                                                                                                  r                                                                          2                                                              -                                                                  (                                                  v                          ·                          r                                                )                                            2                                                                                                                        r                                                              3                                                                                                                                                            d                  ′                                ∼                            ≈                            ⁢                                                -                                      1                    λ                                                  ⁢                                  1                                                            (                                                                                                    (                                                                                          x                                k                                                            -                              x                                                        )                                                    2                                                +                                                                              (                                                                                          y                                k                                                            -                              y                                                        )                                                    2                                                +                                                                              (                                                                                          z                                k                                                            -                              z                                                        )                                                    2                                                                    )                                                              3                      2                                                                                                                                                            ⁢                              {                                  (                                                                                    a                        x                                            ⁡                                              (                                                                              x                            k                                                    -                          x                                                )                                                              +                                                                  a                        y                                            ⁡                                              (                                                                              y                            k                                                    -                          y                                                )                                                              +                                                                                                                                        ⁢                                                                    a                    z                                    ⁡                                      (                                                                  z                        k                                            -                      z                                        )                                                  +                                  v                  x                  2                                +                                  v                  y                  2                                +                                  v                  z                  2                                            )                                                                                        ⁢                                                (                                                                                    (                                                                              x                            k                                                    -                          x                                                )                                            2                                        +                                                                  (                                                                              y                            k                                                    -                          y                                                )                                            2                                        +                                                                  (                                                                              z                            k                                                    -                          z                                                )                                            2                                                        )                                -                                                                                                      ⁢                                                (                                                                                    v                        x                                            ⁢                                              (                                                                              x                            k                                                    -                          x                                                )                                                              +                                                                  v                        y                                            ⁡                                              (                                                                              y                            k                                                    -                          y                                                )                                                              +                                                                  v                        z                                            ⁡                                              (                                                                              z                            k                                                    -                          z                                                )                                                                              )                                2                            }                                                          [                  Equation          ⁢                                          ⁢          9                ]            
where (xk, yk, zk) is position coordinate information of the satellite, (x, y, z) is coordinate information of the BS 110, (ax, ay, az) is an acceleration of the satellite, and (vx, vy, vz) is a velocity vector of the satellite.
There may arise an unexpected propagation delay, however, of a satellite signal while the satellite signal passes through the ionosphere and the troposphere before reaching the MS. The conventional method described above for calculating satellite acquisition information denoted by the aforementioned equations has not compensated for the propagation delay, and has calculated the code phase and Doppler shift information on the basis of a real range between the satellite and the BS 110 (i.e., a transmission range in an ideal state). Therefore, the conventional method has a disadvantage in that the calculated code phase and Doppler shift information may be different from those of a satellite actually observable by the MS.